1. Field of the Invention
The invention relates to a vertical annealing furnace for the continuous bright annealing of metal strip guided through the furnace, comprising a vertically disposed muffle having a strip entry side and a strip exit side, as well as heating means for externally heating the muffle, the muffle having the freedom to expand in the longitudinal direction.
2. Description of the Prior Art
A vertical annealing furnace of this kind is known, inter alia, from Stahl und Eisen, Volume 93, No. 24, of Nov. 22, 1973, pp. 1152-1157. In this case, the muffle comprises a top flange, by means of which it is suspended fixedly in a frame. The muffle can expand freely downwards, in the longitudinal direction, with respect to the rest of the annealing furnace. This possibility of expansion of the muffle is of crucial importance in order to be able to achieve a specific large structural height (e.g. 20 m) of the annealing furnace. This is because, in the case of so-called bright annealing of stainless steel strip, the temperature of the muffle is in the region of 1150.degree. C. At such a high temperature, the expansion of the muffle in the longitudinal direction is very great. If no provisions were then to be made permitting this expansion, this would lead to bending deformations of the muffle, both in the transverse and in the longitudinal direction of the muffle. The strip to be heated passes through the muffle from the bottom to the top. The point where the strip reaches its highest temperature in the muffle is therefore situated at the top of the muffle. In order to guide the strip through the muffle and to be able to hold the strip at a specific stress, a specific strip tension is exerted on the strip, this tension being transmitted to the strip by means of rollers. Downstream of the muffle is situated a cooling section, which, owing to the high final temperature which the strip has reached at the end of the muffle, should be placed directly after the strip exit side of the muffle. As a result, the cooling section is situated entirely or mostly directly above the vertically disposed muffle.
A vertical annealing furnace of the same kind is also known from JP-A-2 282 687 and JP-A-4 225 780. These furnaces are furthermore provided with counterweight mechanisms giving a compression load acting on the lower part of the muffle. Thus downward directed focus caused by the muffle weight and thermal expansion of the muffle are reduced.
A drawback of these known vertical annealing furnaces is that the structural height is limited. There are two reasons for this limitation of the structural height. Firstly, the full weight of the muffle is suspended from the top flange, which means that the maximum permissible stress for the muffle material in the region of the top flange is decisive for the maximum permissible muffle weight suspended therefrom. It may be noted at this point that, in these known annealing furnaces where the strip runs through the muffle from the bottom to the top, the muffle is exposed in its upper section to high temperatures, because this is where the strip to be heated has to reach its final temperature. This high temperature in the upper section of the muffle reduces the maximum permissible tensile stress. In order nevertheless to achieve structural heights of 22-24 m, the thickness of the muffle wall has to increase progressively towards the top, so as not to exceed the tensile stress which is permissible for the muffle material. Secondly, the hottest point of the strip is likewise situated at the top side of the muffle. This most critical point of the strip is as a result subjected to relatively heavy loads due to the inherent weight of the strip situated beneath it and due to the strip tension exerted on the strip. This too entails limitations for the maximum height to which a vertical annealing furnace of this kind can be built. This is because if the furnace is too high, the strip will yield at the weakest point, that is to say at the top side of the muffle. The furnaces as disclosed in JP-A-2 282 687 and JP-A-4 225 780 have the further disadvantage that the counterweight mechanisms counteract the expansion of the muffle. All this places limitations on increasing the production capacity, since building a higher muffle annealing furnace is limited in technical terms.